Raudel Avila, assistant professor of mechanical engineering at Rice University, was determined to develop an improved geometrical and mechanical design for bioelectronics that improves wearability, efficiency, and comfort.
“Conventionally, bioelectronics are typically fabricated on a planar surface that is then deformed to match a required curvature in the body,” Avila said. “I realized we could make the bioelectronic device curve from the start and place it in the locations of interest in the body. This idea is particularly important when you are dealing with very sensitive or fragile skin, such as neonatal intensive care (NICU) or pediatric intensive care (PICU) patients.”
Because this field of research often benefits from cross-collaboration, Avila worked closely with material scientists and biomedical engineers. The team consisting of Sun Soo Kwak (Korea Advanced Institute of Science and Technology), Jean Won Kwak (Stanford University), John A. Rogers (Northwestern University), and Yonggang Huang (Northwestern) developed soft skin-integrated “holey” bioelectronic devices with enhanced mechanics to monitor health metrics in NICU and PICU patients.
The research was reported in the April 2024 issue of Journal of Applied Mechanics.
Bioelectronic devices with soft polymeric encapsulations and internal rigid electronic components result in a mechanically hybrid composite structure, with intrinsically soft mechanics for integration with biological tissues through mechanical compliance. For accurate signal acquisition and sensing on limbs, chest, forehead, and other curved regions of the body, bioelectronic devices are pressed and bent to closely match the skin morphology, causing additional interfacial stresses.
This 3D finite element model shows the stress distribution of a curved biosensor running across the top of a thigh. Credit: Raudel Avila
Avila engineered a mechanical design strategy based on conformal curvature matching, with stiff substrate islands housing commercial electronic components and circuits.
The team used finite element modeling of the skin curvature, encapsulation, and internal electronic layouts to quantify the spatial distribution of the underlying stresses at the skin interface, based on a mismatch curvature angle θ between the device and skin.
“Selected cases of θ = 30 deg and 60 deg can reduce the normal and shear stresses by up to 45 percent and 70 percent, respectively,” Avila said.
They also leveraged geometry and mechanics to design better bioelectronics form factors for applications such as continuous physiological monitoring of NICU and PICU patients.
“We are using traditional concepts from mechanics of materials to ensure that, for the multi-material layout that combines soft elastomers and rigid electronics, low flexural and torsional stiffness can be achieved to accommodate various bending and twisting deformations,” he said.
The biggest research challenge was to systematically understand how geometrical, mechanical, and material parameters affect the interfacial stresses on the skin. These parameters further influence the magnitude of the normal and interfacial stresses. By performing numerical simulations, the team was able to identify how parameters such as the Young’s modulus of the encapsulation elastomers, the bending angle and device curvature, and the placement and layout of the internal electronic layouts influence the distribution of the interfacial stresses during bioelectronic placement on the skin.
The somatosensory pressure (approximately 20 kPa) of skin is the metric the team used to evaluate if the user/patient can feel or sense the device during operation. This threshold value is very important because it is related to the comfort and wearability of the bioelectronic device during operation.
“We do not want devices to get in the way of people or affect the way they perform a task,” Avila said. “One of the biggest surprises of this work was achieving curved bioelectronic designs, with curved angles of up to 60 degrees, that never reach this somatosensory pressure. This means that, even when deformed, the device remains imperceptible to the user, which is very important in clinical settings for NICU and PICU patients.”
Avila and his team of collaborators demonstrated how curvature-matching designs for the bioelectronic–skin interface can reduce the resulting normal and shear stresses generated from device adhesion and skin stretching during dynamic motions.
“Our proposed curvature-matching design strategy can inform the future design of user-specific bioelectronics for integration with complex anatomical structures,” Avila said.
For example, these concepts can be applied for bioelectronics placed on high-curvature locations like pressure monitoring of residual limbs in amputees. “Hopefully, the translational impact of our user-specific/curvature-specific bioelectronics will go beyond hospitals and clinics to enable other applications in different industries.”
Mark Crawford is a technology writer in Corrales, N.M.
